Method of forming a reflective electrode

ABSTRACT

The present invention provides a method of forming a TFT and a reflective electrode having recesses or projections with reduced manufacturing cost and a reduced number of manufacturing steps, and provides a liquid crystal display device to which the method is applied. A photosensitive film  8  is formed on a metal film  7 . Then, remaining portions  81, 82  and  83  are formed from the photosensitive film  8 . Then, the metal film  7  is etched by using the remaining portions  81, 82  and  83  as masks. And then, a photosensitive film  9  and a reflective electrode film  10  are formed without removing the remaining portions  81, 82  and  83.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming a thin filmtransistor (TFT) and a reflective electrode having recesses orprojections in a predetermined area and also to a liquid crystal displaydevice to which the method is applied.

2. Description of Related Art

In recent years, a liquid crystal display device provided with TFTs hasbeen widespread rapidly. When the liquid crystal display device isconstructed as a reflective type or a transflective type, not only a TFTbut also a reflective electrode for reflecting an outer light areprovided in each pixel area. In the liquid crystal display deviceprovided with the reflective electrode, the reflective electrode isoften provided with recesses or projections in order to improve aquality of the image to be displayed.

For example, In the case where the TFT provided in each pixel area hasthe top gate structure, the gate electrode is formed by performing alithographic step before forming the reflective electrode. Next, aphotosensitive film is formed and is patterned into a shape having anumber of recesses or projections and then a reflective electrode isformed thereon. A lithographic step comprises a plurality of stepsincluding an exposure step, a development step and others. Therefore, inthe case where, after forming the gate electrode by performing thelithographic step, the photosensitive film is formed and then patternedinto the shape with a number of recesses or projections, there arises aproblem that the number of manufacturing steps and the manufacturingcosts increase.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention is to provide amethod of forming a TFT and a reflective electrode comprising recessesor projections with a reduced number of manufacturing steps and reducedcosts and a liquid crystal display device to which such method isapplied.

A method of the present invention for achieving the object describedabove is a method of forming a TFT and a reflective electrode havingrecesses or projections in a predetermined area, the method forming atleast a gate electrode and a gate bus by processing a first film, themethod comprising the step of forming the first film, the step offorming a photosensitive film on the first film, the step of patterningthe photosensitive film in such a way that a first portion, a secondportion and a third portion of the photosensitive film remain, the firstportion corresponding to the gate electrode, the second portioncorresponding to the gate bus and the third portion being different fromthe first and second portions, the step of etching the first film byusing the first, second and third portions as masks and the step offorming the reflective electrode in such a way that at least a portionof the reflective electrode is present on at least the third portion ofthe first, second and third portions.

According to the present invention, in the step of patterning aphotosensitive film, the photosensitive film is patterned in such a waythat the first, second and third portions of the photosensitive filmremain (the first portion corresponds to the gate electrode, the secondportion corresponds to the gate bus and the third portion is differentfrom the first and second portions). Further, in the etching step, thefirst film is etched using the first, second and third portions of thephotosensitive film as etching masks, so that gate electrodes and gatebuses can be formed from the first film. After that, in the step offorming a reflective electrode, the reflective electrode is formedwithout removing the first, second and third portions. Therefore, theshape of the reflective electrode can be adjusted depending on the shapeof the first, second and third portions, so that it is possible toprovide the reflective electrode with recesses or projections.

As described above, in the present invention, the photosensitive film isused as etching masks for forming the gate electrode and the gate busand is also used to provide the reflective electrode with the recessesor projections. That is to say, a member used as etching masks forforming the gate electrode and the gate bus and another member forproviding the reflective electrode with the recesses or projections areformed from the same photosensitive film. Therefore, there is no need toform, from different photosensitive films, a member used as etching maskfor forming the gate electrode and the gate bus and another member forproviding the reflective electrode with the recesses or projections, sothat the number of manufacturing steps and the manufacturing costs willbe reduced.

In the present invention, the third portion may comprise a plurality ofparts scattered in the predetermined area or comprise a plurality ofholes in the predetermined area.

By constructing the third portion described above, the reflectiveelectrode can be provided with recesses or projections

Further, in the present invention, an edge of the second portion maycomprise a curved form.

By providing the edge of the second portion with the curved form, thereflective characteristics of the reflective electrode can be furtherimproved.

A liquid crystal display device of the present invention is a liquidcrystal display device comprising a substrate on which a TFT and areflective electrode having recesses or projections are formed in apredetermined area, wherein the reflective electrode is formed by usingthe method as claimed in any one of claims 1 to 4.

Further, in the liquid crystal display device of the present invention,the third portion can be present below the reflective electrode and amaterial of the first film can be present below the third portion.

Furthermore, in the liquid crystal display device of the presentinvention, an edge of at least that portion of the gate bus, which islocated below the reflective electrode, may comprise a curved form.

The invention further relates to a substrate on which a TFT and areflective electrode having recesses or projections are formed in apredetermined area, the substrate further comprising:

a first film patterned to comprise a first portion, a second portion anda third portion, the first portion forming a gate electrode of the TFT,the second portion forming a gate bus, the third portion being differentfrom the first and the second portion, and at least the third portionbeing formed in the predetermined area;

provided on the substrate provided with the first film, a photosensitivefilm patterned to comprise a first photosensitive portion, a secondphotosensitive portion and a third photosensitive portion, the firstportion being masked by and aligned with the first photosensitiveportion, the second portion being masked by and aligned with the secondphotosensitive portion, and the third portion being masked by andaligned with the third photosensitive portion;

provided on the substrate provided with the photosensitive film, areflective electrode covering at least the third photosensitive portionof said first, second and third photosensitive portions and an area ofthe substrate adjacent the third photosensitive portion to form thereflective electrode having recesses or projections in a predeterminedarea and a liquid crystal display device comprising such.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a TFT substrate assembly in which TFTs andreflective electrodes have been formed in accordance with an embodimentof the method according to the present invention.

FIG. 2 is a cross-sectional view of the assembly taken along a line I—Iof FIG. 1.

FIG. 3 is a plan view of a substrate immediately after a gate insulatingfilm 6 has been formed thereon.

FIG. 4 is a cross-sectional view of the substrate taken along a lineII—II of FIG. 3.

FIG. 5 is a cross-sectional view of the substrate on which the metalfilm has been formed.

FIG. 6 is a cross-sectional view of the substrate on which thephotosensitive film has been formed.

FIG. 7 is a plan view of the substrate after the photosensitive film 8has been patterned.

FIG. 8 is a cross-sectional view of the substrate taken along a lineIII—III of FIG. 7.

FIG. 9 is a plan view of the substrate after the metal film 7 has beenetched.

FIG. 10 is a cross-sectional view of the substrate taken along a lineIV—IV of FIG. 9.

FIG. 11 is a cross-sectional view of the substrate on which thephotosensitive film has been formed.

FIG. 12 is a cross-sectional view of the substrate on which thereflective electrode film 10 has been formed.

FIG. 13 is a cross-sectional view of the conventional substrate on whichTFTs and reflective electrodes have been formed.

FIG. 14 is a plan view of the substrate on which remaining portions 830have mesh pattern.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Hereinafter, an embodiment of the present invention will be described.

FIG. 1 is a plan view of a TFT substrate assembly in which TFTs andreflective electrodes have been formed in accordance with an embodimentof the method according to the present invention. FIG. 2 is across-sectional view of the assembly taken along a line I—I of FIG. 1.

Hereinafter, a method of manufacturing the TFT substrate assembly shownin FIGS. 1 and 2 will be described with reference to FIGS. 3 to 12.

FIG. 3 is a plan view of a substrate immediately after a gate insulatingfilm 6 has been formed thereon. FIG. 4 is a cross-sectional view of thesubstrate taken along a line II—II of FIG. 3.

As shown in FIG. 4, source electrodes 2, drain electrodes 3, sourcebuses 4, semiconductor layers 5 of a-Si:H (hydrogenated amorphoussilicon) for example and the gate insulating film 6 are formed on thesubstrate 1. A rectangular window 6 a (see FIG. 3) for exposing a partof each drain electrode 3 is formed in the gate insulating film 6. Afterforming the gate insulating film 6, a metal film for the gate electrodesand gate buses is formed (see FIG. 5).

FIG. 5 is a cross-sectional view of the substrate on which the metalfilm has been formed.

For example, an Al (aluminum) film can be used as the metal film 7.After forming the metal film 7, a photosensitive film is formed withoutpatterning the metal film 7 (see FIG. 6).

FIG. 6 is a cross-sectional view of the substrate on which thephotosensitive film has been formed.

The photosensitive film 8 directly contacts the metal film 7 in thisembodiment but a further film may be interposed between thephotosensitive film 8 and the metal film 7. After forming thephotosensitive film 8 as shown in FIG. 6, the photosensitive film 8 ispatterned by exposing the photosensitive film 8 to light and developingit (see FIGS. 7 and 8).

FIG. 7 is a plan view of the substrate after the photosensitive film 8has been patterned. FIG. 8 is a cross-sectional view of the substratetaken along a line III—III of FIG. 7.

A part of the photosensitive film 8 is removed by developing thephotosensitive film 8, so that portions 81, 82 and 83 remain. Theportion 81 remains on a portion of the metal film 7 corresponding to thegate electrode 71 described later (see FIG. 10). The portion 81 remainsso as to extend in a y direction (see FIG. 7). The portion 82 remains ona portion of the metal film 7 corresponding to the gate bus 72 describedlater (see FIG. 10). The portion 82 remains so as to extend in an xdirection (see FIG. 7). Edges 82 a of the remaining portion 82 areformed in the wavy shape in the plan view (see FIG. 7). The reason forforming the edges 82 a in the wavy shape will be described later. Theportions 81 and 82 remain connected to each other without beingseparated. The remaining portions 83 each have a generally hemisphericalshape and are distributed on the substrate. The remaining portions 83play a role in providing a reflective electrode 100 described later (seeFIG. 2) with projections 10 c. The portions 83 remain separated from theremaining portions 81 and 82. Next, the metal film 7 will be etchedusing the remaining portions 81, 82 and 83 as etching masks (see FIGS. 9and 10).

FIG. 9 is a plan view of the substrate after the metal film 7 has beenetched. FIG. 10 is a cross-sectional view of the substrate taken along aline IV—IV of FIG. 9.

When the metal film 7 is etched, the gate electrode 71 is formed belowthe remaining portion 81 while the gate bus 72 is formed below theremaining portion 82 as shown in FIG. 10. Since the edges 82 a of theremaining portion 82 have the wavy shape, edges of the gate bus 72 arealso formed in the wavy shape. A metal piece 73 made of the material ofthe metal film 7 remains below each remaining portion 83. Since eachremaining portion 83 is separated from the remaining portions 81 and 82,the metal piece 73 remaining below each remaining portion 83 isseparated from the gate electrode 71 and the gate bus 72. In this way,the gate electrodes 71, the gate buses 72 and the metal pieces 73 areformed below the remaining portions 81, 82 and 83, respectively. In thisembodiment, the portions 83 are remained in order to provide thereflective electrode 100 described later (see FIG. 2) with projections10 c. Therefore, when the metal film 7 is etched, the remaining portions83 act as etching masks, so that a plurality of metal pieces 73 areformed. However, it should be noted that the remaining portions 83 arenot remained in order to form the metal pieces 73 and that each metalpiece 73 is a mere by-product which is formed because of the existenceof the remaining portion 83 and the metal piece 73 does not contributeto the operation of the TFTs. In this embodiment, although only the gateelectrodes 71, the gate buses 72 and metal pieces 73 are formed from themetal film 7, electrodes each for providing a respective pixel area witha storage capacitance may additionally be formed from the metal film 7.

After etching the metal film 7 as described above, a photosensitive filmis formed (see FIG. 11).

FIG. 11 is a cross-sectional view of the substrate on which thephotosensitive film has been formed.

The photosensitive film 9 comprises a window 9 d for exposing a part ofthe drain electrode 3. Since the remaining portions 81, 82 and 83 havebeen formed below the photosensitive film 9, the photosensitive film 9is formed at its surface with projections 9 a, 9 b and 9 c, reflectingthe shape of the remaining portions 81, 82 and 83. By these projections9 a, 9 b and 9 c, a gentle undulation is formed over the entire surfaceof the photosensitive film 9. After forming the photosensitive film 9, areflective electrode film is formed so as to cover the photosensitivefilm 9 (see FIG. 12).

FIG. 12 is a cross-sectional view of the substrate on which thereflective electrode film 10 has been formed.

Since the window 9 d (see FIG. 11) has been formed in the photosensitivefilm 9, the reflective electrode film 10 is connected to the drainelectrode 3. Since the photosensitive film 9 has been formed below thereflective electrode film 10, the reflective electrode film 10 reflectsthe shape of the projections 9 a, 9 b and 9 c (see FIG. 11), so that thereflective electrode film 10 have projections 10 a, 10 b and 10 c. Bythese projections 10 a, 10 b and 10 c, a gentle undulation is formedover the entire surface of the reflective electrode film 10. Afterforming the reflective electrode film 10 as described above, thereflective electrode film 10 is divided into pixel areas by performingthe lithographic step, so that a reflective electrode 100 (shown byhatching) comprising projections 10 a, 10 b and 10 c is formed in eachpixel area as shown in FIGS. 1 and 2. By providing the reflectiveelectrode 100 with the projections, it becomes possible to provide thereflective electrode 100 with a desired reflective characteristic. Inthis embodiment, each reflective electrode 100 is formed so as to extendnot only on a region in which the remaining portions 83 have been formedbut also on a region in which the remaining portions 81 and 82 have beenformed (see FIG. 2), but the reflective electrode 100 may be formed, forexample, only on the region in which the remaining portions 83 have beenformed.

In this embodiment, the portions 81, 82 and 83 are remained bypatterning the photosensitive film 8 and the metal film 7 is etchedusing the remaining portions 81, 82 and 83 as etching masks. By makingthe remaining portions 81 and 82 act as etching masks, the gateelectrode 71 and the gate bus 72 are formed, and by making the remainingportion 83 act as an etching mask, the metal piece 73 is formed. Sinceeach remaining portion 83 is separated from the remaining portions 81and 82, each metal piece 73 is separated from the gate electrode 71 andthe gate bus 72. The remaining portions 81, 82 and 83 are not removedand the photosensitive film 9 is formed so as to cover the remainingportions 81, 82 and 83, and then the reflective electrode film 10 isformed on the photosensitive film 9. The reflective electrode film 10follows the shape of the photosensitive film 9 and thus has theprojections 10 a, 10 b and 10 c. As described above, in this embodiment,when the photosensitive film 8 is etched, each remaining portion 83which plays a role in providing the reflective electrode 100 withprojections is also formed in addition to the remaining portions 81 and82 which play roles as etching masks for forming the gate electrode 71and the gate bus 72, respectively. Therefore, there is no need to form,from different photosensitive films, a member used as etching mask forforming the gate electrode 71 and the gate bus 72 and another member forproviding the reflective electrode 100 with projections, so that thenumber of manufacturing steps and the manufacturing costs will bereduced. Hereinafter, the manner in which the manufacturing step numberand the manufacturing costs are reduced will be described in comparisonwith the conventional way.

FIG. 13 is a cross-sectional view of the conventional substrate on whichTFTs and reflective electrodes have been formed.

Conventional way requires that, after a metal film for gate electrodesand gate buses is formed, the metal film is patterned by performing alithographic step to form the gate electrodes 71 and the gate buses (notshown), and after a photosensitive film is formed, a plurality ofportions 800 are remained by exposing the photosensitive film to lightand developing it. Therefore, it is required that an etching mask forpatterning the metal film and the remaining portions 800 for providingthe reflective electrode 100 with projections must be formed fromdifferent photosensitive films. In contrast, in that embodimentaccording to the present invention, the remaining portions 81 and 82which play roles as etching masks for forming the gate electrode 71 andthe gate bus 72, respectively, and the remaining portion 83 which playsa role in providing the reflective electrode 100 with projections areformed from the same photosensitive film 8. Therefore, it will beappreciated that the number of manufacturing steps and the manufacturingcosts are reduced in the present invention.

In this embodiment, the edges 82 a of the remaining portion 82 is formedin the wavy shape. By forming the edges 82 a in the wavy shape, theshape of the projections 10 b of the reflective electrode 100 reflectsthe wavy shape of the edges 82 a, so that the reflective characteristicsof the reflective electrode 100 can be further improved. It is notedthat the edges 82 a of the remaining portions 82 may have a straightshape instead of the wavy shape if the reflective electrode can obtain adesired reflective characteristic.

In this embodiment, the metal film 7 is of the single layer structurebut the film 7 may be of the multilayer structure consisting of aplurality of layers such as Al (aluminum) film/Mo (molybdenum) film. Byforming the film 7 in the multilayer structure, it becomes possible thatthe gate electrode 71 and the gate bus 72 comprise a multilayerstructure having two or more layers.

In this embodiment, the remaining portion 83 is formed in thehemispherical shape but the portion 83 may be formed in a shape otherthan the hemispherical shape. Hereinafter, one example in which theremaining portion is formed in a shape other than the hemisphericalshape is described with referencet to FIG. 14.

FIG. 14 is a plan view of the substrate on which remaining portions 830have mesh pattern.

In order to form the remaining portion 830 in the mesh pattern, afterthe photosensitive film 8 is formed (see FIG. 6), a plurality of holes830 a may be formed in the photosensitive film 8. In this way, theremaining portion 830 having the mesh pattern (shown by hatching) isformed in addition to the remaining portions 81 and 82. In the casewhere the remaining portion 830 having the plurality of holes 830 a isformed instead of making the plurality of hemispherical shaped remainingportions 83 distribute, the reflective electrode 100 reflects the shapeof the plurality of holes 830 a, so that the electrode 100 has recessesinstead of the projections 10 c (see FIG. 12). By providing thereflective electrode 100 with the recesses, it is also possible toprovide the reflective electrode 100 with a desired reflectivecharacteristic. It is noted that the remaining portion 830 is formed soas to be separated from the remaining portions 81 and 82 in FIG. 14 butthe remaining portion 830 may be formed so as to be connected, forexample, to the remaining portion 82.

According to the present invention, there are provided a method offorming a TFT and an electrode comprising recesses or projections withreduced number of manufacturing steps and reduced costs and a liquidcrystal display device to which such method is applied.

What is claimed is:
 1. A method of forming a TFT and a reflectiveelectrode having recesses or projections in a predetermined area, saidmethod forming at least a gate electrode and a gate bus by processing afirst film, said method comprising the steps of: forming said firstfilm; forming a photosensitive film on said first film; patterning saidphotosensitive film in such a way that a first portion, a second portionand a third portion of said photosensitive film remain, said firstportion corresponding to said gate electrode, said second portioncorresponding to said gate bus and said third portion being differentfrom said first and second portions; etching said first film by usingsaid first, second and third portions as masks; and forming saidreflective electrode in such a way that at least a portion of saidreflective electrode is present on at least said third portion.
 2. Amethod as claimed in claim 1, wherein said third portion comprises aplurality of parts scattered in said predetermined area.
 3. A method asclaimed in claim 1, wherein said third portion comprises a plurality ofholes in said predetermined area.
 4. A method as claimed in claim 1,wherein an edge of said second portion comprises a curved form.
 5. Themethod as claimed in claim 2, wherein an edge of the second portioncomprises a curved form.
 6. The method as claimed in claim 3, wherein anedge of the second portion comprises a curved form.